This invention relates to a mold closing unit of an injection molding machine which includes a stationary mold clamping plate and a movable mold clamping plate slidable along parallel guide rods relative to the stationary plate, a movable mold affixed to the movable plate, a mating fixed mold die affixed to the stationary plate, and the unit comprising a fixed, hydraulic mold closure retention cylinder, and a hydraulic traveling cylinder including a piston operable within the retention cylinder and being connected with the movable die.
In known injection molding machines, the injection cycle is effected in such a manner that, while the mold is closed, plastic material is injected into the mold cavity formed by the two halves of the mold, the mold is opened, the plastic molded element is ejected with the opening of the mold and the mold is again closed for the production of another plastic molded element. The closing of the mold is effected from a distance corresponding to the maximum distance between the mold clamping plates in such a manner that the movable mold clamping plate is initially accelerated to the maximum speed and then is decelerated when a specified distance is reached from the stationary mold clamping plate depending on the height or thickness of the clamped injection mold so that the mold is closed below the creep rate. Subsequently, the mold closure retention cylinder is hydraulically acted upon to effect the required mold closure retention force (closing force).
The ejection of the plastic molded element is effected during the opening process. However, the plastic molded element sometimes remains stuck to or in one-half of the mold and thereby blocks the closing of the mold. Since the closing of the mold takes place under a reduced hydraulic pressure in its final phase in order to protect the mold cavity against damage, the movable mold clamping plate with the movable die mold clamped thereto remains in a position blocked by the plastic molded element. In order to sense such an error, which is referred to herein as a mold closure security error, a minimum time interval was formerly specified within which the closing position of the mold was to be achieved in the final phase. Thus, as this time interval was exceeded, a malfunction signal was emitted as an indication of a mold closure error. It can be seen that, however, with a closing of the mold below the creep rate, i.e. with a reduced hydraulic pressure, a malfunction indication can also be effected when, for example, the lubrication on the guide rails of the molding machine deteriorates such that the movable mold clamping plate is caused to stick due to increased friction. Also, a bent guide rail can trigger such an error indication. Thus, factors other than a mold closure security error can trigger an error indication. Moreover, it is difficult to indicate mold security errors in a time-dependent fashion such as, for example, when a longer mold closure security path is specified for a simultaneous extension of the height of the mold when changing molds, such that this longer mold closure security path will result in the specification of a longer minimum time for the closing of the mold in the final phase.